Isomerization of cyclobutane ligands in the solid state and solution

Cyclobutane compounds are a class of compounds that can be conveniently synthesized in the solid state by employing crystal engineering principles. The rctt-isomer (or the syn-dimer) is the most common form of any cyclobutene compound that is obtained in the solid state by the photochemical 2 + 2] cycloaddition reactions. However, these rctt-isomers can be converted to other less accessible forms, under some special conditions. Isomerization of cyclobutane compounds thus plays an important role in synthetic chemistry. Such isomerization of cyclobutane compounds have been reported in organic salts, metal complexes, coordination polymers and metal-organic frameworks. In this review, these fascinating examples of isomerization that occur both in the solid state and solution phase have been discussed.